How Do We Know?

How do we know the universe is expanding? Well, for a long time, we didn't know. Although the universe is expanding all around us, the expansion happens over such a large scale that it's very hard to notice it from Earth.
The expansion of the universe was discovered in 1929 by American astronomer Edwin Hubble. He used the 100-inch telescope on Mount Wilson in California - at the time, the largest telescope in the world - to measure the distances to other galaxies.

He combined his distances with another scientist's measurements of the speed with which galaxies were moving away from us. He created a graph of distance vs. velocity for the universe, just like the graph you created for the balloon. Looking at his graph, he realized that the universe must be expanding. Today, we call such a graph a Hubble Diagram.

The Universe Project

You don't just have to take our word for it, though - you can make your own Hubble Diagram to prove to yourself that the universe is expanding! In the pages that follow, you will retrace Hubble's steps to make one of the most important discoveries of 20th century astronomy.

You will look at a few galaxies in the SkyServer database. This database contains actual data that has been collected by many astronomers. It provides astronomers with a catalogue of objects that are known to exist in the universe. So far, there are nearly 90 million stars and galaxies in this catalogue! The database is being used by astronomers to make the largest, most detailed map of the universe ever made. SkyServer makes it available to the public, so you can explore the mysteries of the universe.

Your first step will be to look at how bright these galaxies are in order to get a rough idea of how far away from Earth they are. Next, you will use these distances, along with SkyServer's measurements of how red the galaxies are, to make a simple Hubble Diagram to show the universe's expansion.